A lambie is a fellow friend who loves L.A.M.B. (Clothing and bag line by Gwen Stefani)

This term is usually used in a friendly way. Sometimes, the person will not know the name of the individual wearing L.A.M.B., so 'lambie' is just a friendly term to indicate that they also love L.A.M.B. and find it cool that you're wearing an item of it.